The capability to rapidly and accurately determine the physical location of a mobile communication device can be of great benefit in a variety of applications. For example, in a military or policing context, it is desirable to keep track of the position of personnel to increase overall safety of the personnel involved and to provide improved situational awareness for commanders. It is also desirable to track the location of selected items such as lap top computers, automobiles, shipping containers, among other things. While Global Positioning System (GPS) technology for obtaining accurate position information has proliferated in recent years, there are instances where the necessary line of sight to one or more satellites for GPS operation is obscured or entirely unavailable. For example, in a military context, personnel may be operating both outdoors and indoors. Those indoors may simply not have the benefit of receiving GPS signals, thus making it difficult to know their precise location. Likewise, an automobile parked in a garage or a shipping container in a warehouse would not have the benefit of GPS signals. Accordingly, position tracking of such personnel or items is not possible when relying exclusively on GPS.
To address the limitations of GPS technology, some radio transceivers rely on a location determination approach that is based on Time of Arrival (TOA) of packets or messages exchanged between radios. More specifically, to determine the position of a mobile communication device, the device receives multiple timing signals from known locations. Knowing how long the messages take to be received, it is possible to calculate a range between the device and the location of the transmitters providing the timing signals. The mobile communication device can then compute its position using triangulation, assuming signals from multiple transmitters can be received.
In one well-known implementation of such a ranging system, request to send (RTS) and clear to send (CTS) packets or messages that are exchanged between radios in accordance with selected protocols are leveraged to acquire timing information. More specifically, a source radio sends an RTS packet to a destination radio, and the destination radio replies with a CTS packet if the destination radio can accept the message to be supplied by the source radio. Upon receipt of the CTS packet, the source radio sends a message (MSG) packet. The destination radio then sends back an acknowledgement (ACK) if it successfully received the message. Radios operating in accordance with such a protocol can use the acquisition part of the RTS and CTS packets to determine when synchronization occurs. This synchronization information can then be used to determine the range between the two transceivers by timing how long it takes the CTS to come back to the source radio, i.e., the round trip time can be calculated.
More sophisticated radio transceivers may also use special ranging transmissions to more accurately determine the range between the radios. Such radios are described in, e.g., U.S. Pat. No. 6,453,168 to McCrady et al., which is incorporated herein by reference. Such a system employs dedicated TOA packets that contain a specially designed TOA reference bit pattern that operates with more robust synchronization detectors. The TOA bit patterns are usually very slow, long, and highly ideal correlator patterns compared to that for a high data rate radio system. These TOA reference patterns are used by the receiver to determine the synchronization time more accurately by effectively interpolating when the synchronization peak occurs finer than the clock's resolution. The TOA packets are used to range between several transceivers. This allows the transceivers, as in the more conventional approaches, to determine their relative positions. This information may then be used to form a situational view of the locations of the receivers.
As described in U.S. Pat. No. 6,453,168, conventional RTS and CTS messages can be modified to support the ranging messaging scheme. In effect, conventional hardware for handling RTS and CTS exchanges is leveraged to also capture “RTS” and “CTS” messages that contain the more sophisticated TOA synchronization data or bit patterns.
The aforementioned techniques either provide a low accuracy answer (when relying exclusively on the acquisition portion of conventional RTS and CTS messages) or cause a network throughput “hit” in that additional, modified, “RTS” and “CTS” messages are being exchanged over the network, thereby consuming bandwidth that might otherwise be used for voice/data communications. More specifically, the ranging measurement provided by the conventional synchronization technique is coarse—it provides a relatively inaccurate measurement that may not provide sufficiently useful situational awareness. In the more sophisticated approach, the special TOA packets cause a reduced data throughput. When the TOA packets are being transmitted, no network data can be transmitted. Situational awareness needs high accuracy (<3 meters) and this means that a radio may need to send several TOA packets to several radios in order to triangulate its position. Additionally, all radios need to do this, resulting in even less overall bandwidth that is available for voice/data communications.